In simple renal ectopia, the kidney and ureter are on their expected sides of the spine. The ectopic kidney is most commonly located in the pelvis. It is usually small and malrotated and frequently has a dysmorphic configuration, such as a pancake, disk, or lump shape (Fig. 8.6).

In crossed fused ectopia, both kidneys are located on the same side of the spine. The crossed ectopic kidney is smaller than the normal kidney and malrotated, and it usually lies caudal to the orthotopic kidney. Typically, the upper pole of the ectopic kidney is fused to the lower pole of the orthotopic unit (Fig. 8.7). Both distal ureters are normally located. The ureter
draining the ectopic kidney returns enters the bladder at the contralateral trigone, whereas the ureter of the orthotopic component enters the ipsilateral trigone. There is an increased incidence of multicystic dysplasia, hydronephrosis, infection, and calculus formation in the crossed unit.

Horseshoe kidney is the most common renal fusion anomaly. Typically, the lower poles of the kidneys are fused and connected by an isthmus of tissue. Associated anomalies are common and include uteropelvic junction obstruction, vesicoureteral reflux, duplicated collecting systems, and renal dysplasia. There is an increased risk of infection and stone formation. There also is a slightly increased incidence of Wilms tumor and renal cell carcinoma. Imaging findings include anteriorly located renal pelves, medial orientation of the lower poles of the kidneys, and an isthmus of tissue crossing the midline anterior to the great vessels (Fig. 8.8).

Simple cysts are rare in children, with an incidence of less than 1%. Pathologically, they are unilocular masses lined by a single layer of flattened epithelium and contain clear serous fluid. Simple cysts arise in the renal cortex, do not communicate with the collecting system, and are more often solitary than multiple. They are usually asymptomatic and
detected during imaging examinations performed for other indications. However, very large cysts can present as palpable abdominal masses. Sonographic, CT, and MR features include a smooth, round shape; imperceptible walls; and homogenous fluid contents. The echogenicity, attenuation value, and signal intensity are equal to that of water. The cysts are avascular and do not enhance after administration of intravenous iodinated contrast medium or gadolinium (Fig. 8.18).

Calyceal diverticula need to be distinguished from simple cysts. The former are cystic spaces that are lined by transitional epithelium and communicate with the collecting system through a narrow orifice. Most diverticula demonstrate some filling with contrast-enhanced urine, usually best seen on delayed CT scans.

Multicystic dysplastic kidney is a nonhereditary developmental anomaly. Histologically, there are noncommunicating cysts of various sizes, separated by tissue containing primitive dysplastic elements. Contralateral renal abnormalities are common and include vesicoureteral reflux, ureteropelvic junction obstruction, primary megaureter, and ureteropelvic disruption. Most cases are diagnosed on prenatal sonography.

The sonographic features of the classic multicystic dysplastic kidney are multiple cysts of varying size with a random distribution, absence of communication between the cysts, no discernible renal pelvis or sinus, and absent or dysplastic renal parenchyma (Fig. 8.19). Calcifications may be noted in the cyst walls or in the septations between cysts. The contralateral kidney usually shows compensatory hypertrophy. Rarely, multicystic dysplastic kidney may appear as a solitary cystic mass in the renal fossa.

Renal scintigraphy confirms the diagnosis of multicystic dysplastic kidney by showing absence of function. It should be noted that severe hydronephrosis and the multicystic dysplastic kidney may be indistinguishable by scintigraphy.

Multicystic dysplastic kidney is treated nonoperatively, because the natural history is spontaneous decrease in size during the first year of life (12). Surgery is performed if there is respiratory distress because of thoracic compression by the large cystic kidney. Patients treated conservatively are followed with serial sonograms, because of a possible increased risk of Wilms tumor.

Polycystic renal disease can be classified into autosomal recessive and autosomal dominant types. In the autosomal recessive form of disease, previously known as infantile polycystic kidney disease, the kidneys are markedly enlarged but retain their reniform shape. Histologic studies show that these cysts are predominantly dilated collecting tubules, which are radially arranged, extending from the medulla to the subcapsular cortex. The nephrons are normal or minimally altered. Virtually all patients with autosomal recessive cystic disease have hepatic abnormalities, characterized by biliary ductal ectasia and periportal fibrosis. The clinical features of autosomal recessive polycystic disease are dependent on the age of presentation. Infants tend to have more severe renal disease and milder hepatic disease. Patients who survive the neonatal period and come to clinical attention later in life have milder renal disease and more severe hepatic disease (usually portal hypertension).

In the neonate, sonography typically shows bilateral renal enlargement, diffusely increased parenchymal echogenicity, and poor corticomedullary differentiation (13) (Fig. 8.20). Occasionally, there may be a thin rim of hypoechoic parenchyma at the periphery that is thought to be compressed cortex. Cortical cysts also may be evident. On CT, the kidneys are enlarged and show a striated pattern of contrast material excretion, representing contrast stasis in dilated tubules (Fig. 8.21). The renal parenchyma is hypointense on T1-weighted MR images and hyperintense on T2-weighted images. Dilated bile ducts may be seen on sonography, CT, and MRI.

The kidneys in older children may be normal-sized or enlarged (13) (Fig. 8.22). The echogenicity is increased, but it often is limited to the medullary area. There usually is some parenchymal striation on CT but less than that in the neonate. Nodular hepatic margins, splenomegaly, and esophageal varices, typical of cirrhosis, are other common findings.

Autosomal dominant polycystic disease, previously known as adult polycystic renal disease, is a hereditary disorder resulting from an abnormality on the short arm of chromosome 16. Pathologically, multiple cysts of varying size are present in the cortex and medulla with islands of normal parenchyma interspersed between the cystic areas. The cysts communicate with the nephrons as well as the collecting tubules. Cysts are also common in the liver, but there usually is no significant periportal hepatic fibrosis. Cysts also may be found in the pancreas, lungs, spleen, ovaries, testes, and seminal vesicles. Clinical findings include palpable abdominal masses in neonates and hypertension, flank pain, and hematuria in children and adolescents.

In the neonate, sonography demonstrates enlarged, diffusely echogenic kidneys. The sonographic findings are similar to those of autosomal recessive polycystic disease, and the diagnosis needs to be based on family history or renal biopsy.

In older children and adolescents, the kidneys may be of normal size or enlarged and have lobulated or smooth borders. Multiple cysts are seen throughout the full thickness of the renal parenchyma. They have the typical appearance of simple cysts on sonography, CT, and MRI (Fig. 8.23).

Glomerulocystic disease is a rare sporadic renal disorder characterized by cystic dilatation of the Bowman space, with a variable degree of dilatation of the proximal collecting tubules. Periportal hepatic fibrosis, bile duct hyperplasia and dilatation, and hepatic cysts also can be seen. Sonographic findings are similar to those of autosomal recessive and dominant polycystic disease in the neonate (Fig. 8.24). The diagnosis is usually established by renal biopsy.

Medullary sponge kidney, also known as medullary tubular ectasia, is a sporadic rather than an inherited disorder. Pathologically, there is cystic dilatation of the collecting tubules in the area of the renal pyramids. The condition is asymptomatic, unless complications, such as infection or urolithiasis, occur. Uncomplicated tubular ectasia is difficult to recognize on imaging studies. Urolithiasis is seen as hyperechoic renal pyramids on sonography and as small calcifications on CT (see section on calcifications).

Juvenile nephronophthisis and medullary cystic disease are hereditary diseases characterized by polyuria, polydipsia, salt-wasting, severe anemia, and ultimately renal failure. Pathologically, the kidneys are small or normal-sized, and cysts are present in the medulla and at the corticomedullary junction. Juvenile nephronophthisis is inherited as an autosomal recessive trait and becomes symptomatic in childhood or adolescence. Medullary cystic disease is inherited as an autosomal dominant trait and usually becomes symptomatic in adulthood. Imaging studies show small kidneys, loss of corticomedullary differentiation, and a variable number of medullary or corticomedullary cysts (Fig. 8.25).

Tuberous sclerosis is an autosomal dominant disorder resulting from a defect on chromosome 9. Classic clinical features are seizures, mental retardation, and cutaneous lesions. Von Hippel-Lindau disease is due to an error on the short arm of chromosome 3. Retinal and central nervous system hemangioblastomas, pancreatic cysts and islet cell tumors, and pheochromocytoma are common clinical findings. Renal cysts are common in both disorders, and they are usually bilateral (Fig. 8.26). Correlation with family history and clinical findings usually permits the correct diagnosis to be established.

Cysts can also be found in a variety of other syndromes, including Turner syndrome, Meckel syndrome (microcephaly, polydactyly, posterior encephalocele), Jeune asphyxiating thoracic dystrophy (small chest, respiratory failure, renal dysplasia), oral-facial-digital syndrome, Zelweger syndrome (cerebrohepatorenal syndrome), and Beckwith-Wiedemann syndrome. Their imaging characteristics are identical to those of simple cysts. Correlation with clinical findings usually permits the correct diagnosis.

Wilms tumor (nephroblastoma) is the most common renal malignancy in children (14,15,16). Affected children usually present before 5 years of age (mean age, 3 years). The common clinical finding is a palpable abdominal mass. Other findings include abdominal pain, fever, microscopic or gross hematuria, and hypertension. Occasionally, patients present with the Budd-Chiari syndrome secondary to hepatic vein invasion by tumor. Associated abnormalities, such as Beckwith-Wiedemann syndrome, nonfamilial aniridia, congenital hemihypertrophy, and Drash syndrome (male pseudohermaphrodism and renal failure)
occasionally prompt evaluation. Approximately, 10% of children have metastatic disease at presentation. Metastases are characteristically to the lungs (10% to 20%) and less frequently to the liver. Bilateral synchronous tumors occur in 5% to 10% of patients. Wilms tumor may occur in extrarenal sites, such as the retroperitoneum, inguinal region, pelvis, or thorax. These tumors are not usually associated with a primary renal tumor.